Risk factors for haematological malignancies : immune-mediated diseases, body mass index and magnetic fields

Sammanfattning: Haematological malignancies constitute approximately 7% of all cancer cases in Sweden, and leukaemia is in fact the most common malignancy among children. The few established risk factors, such as ionising radiation and immunodeficiency, only explain a small number of the cases. The overall aim of this thesis was to examine the association between some potential risk factors and the development of haematological malignancies. Risk factors investigated were diseases affecting the immune system, excess body weight and exposure to magnetic fields. We investigated the risk of childhood leukaemia following exposure from magnetic fields in infant incubators in a nationwide case-control study. In Sweden, 752 cases occurred among all live-bom infants between 1973 and 1989, followed until the end of 1989. Magnetic field exposure was thoroughly assessed using data from medical records on length of infant incubator treatment, as well as measurements of magnetic field levels inside each incubator model. We found no association between magnetic field exposure and childhood leukaemia overall. However, elevated risk estimates were found in subgroup analyses of acute lymphoblastic leukaemia in the 5-9 year age group. Random variation is a likely explanation for the latter findings. An alternative interpretation is that a relatively long induction period is required. We examined the influence of allergic conditions and autoimmune diseases on risk of haematological malignancies in two studies based on different materials. First, we conducted a cohort study including 16,539 Swedish twins with prospectively collected data on allergic conditions from questionnaires. Haematological malignancies registered between 1969 and 1999 were identified by cross-linkage to the Swedish Cancer Registry. A possible increase in risk of leukaemia was suggested in relation to hives, while eczema during childhood was associated with a tendency towards an elevated risk of non-Hodgkin's lymphoma (NHL). The findings are, however, based on a small number of cases. Second, in a nationwide registrybased case-control study comprising 39,908 cases of haematological malignancies that occurred between 1987 and 1999, and 149,344 controls, occurrence of asthma and autoimmune disease was assessed from hospital discharge diagnoses. Asthma did not affect the risk of developing haematological malignancies; if anything, the results in the registrybased study indicated decreased risks. We found positive associations between Thl-mediated diseases where chronic inflammation is part of the pathogenesis, such as psoriasis and Sjögren´s syndrome, and haematological malignancies. Our findings suggest that chronic autoimmunity and immune stimulation contribute to the risk of developing haematological malignancies. The relationship between overweight, obesity and haematological malignancies was studied in a prospective cohort study based on Swedish and Finnish twin cohorts. In total 70,194 persons were followed in the National Cancer Registries for about three decades. Our results indicate that obesity may be involved in the pathogenesis of myeloma, and excess weight possibly in the pathogenesis of chronic myeloid leukaemia and Hodgkin's lymphoma. Risks of developing chronic lymphocytic leukaemia and NHL were not affected by excess weight.